Contaminated air supplies are of concern in many economic sectors, such as commercial horticulture, as well as to scientists studying growth and metabolism of plants, animals, or microorganisms. Ethylene is produced by and has effects on all of them. Ethylene is widely recognized as a plant growth regulator and levels less than 50 ppb in air can alter plant metabolism. For instance, the role of ethylene in the ripening of climacteric fruits is well documented. The post-harvest storage life of climacteric fruits can be extended by removing ethylene, evolved by the produce and from other sources, from the storage atmosphere. Our invention provides a reliable, efficient, and safe method for obtaining hydrocarbon free air for other purposes as well.
Exhaust gases may also include at very low concentrations, harmful low molecular weight organic compounds such as hydrocarbons and/or hydrocarbons containing heteroatoms. Ethylene oxide is an example of a contaminant in the medical field. It is commonly used as a sterilizing agent in a sterilizing gas which includes as carriers various types of halocarbons (Freon.RTM.) or other environmentally friendly carriers. Ethylene oxide is also an important industrial chemical and can be a pollutant. Its major use is as an intermediate in the production of various chemicals. It may also be used in food sterilization.
A variety of oxidative and adsorptive approaches to removing organic compounds present as impurities in air supplies have been proposed. Adsorbent traps are often less effective, as they have finite binding capacities and they frequently require the handling and/or disposal of toxic materials. For most applications, oxidative methods are preferred, including reaction with ozone, atomic oxygen, potassium permanganate or metal catalysts. Ozone is a powerful oxidant but it is highly corrosive and toxic to man at low concentrations. Atomic oxygen is more reactive than ozone towards ethylene but this approach, like reaction with ozone, requires specialized equipment and trained personnel. Although permanganate is non-volatile, a large surface area is required to remove trace amounts of ethylene from air supplies. Permanganate is normally not reusable and requires special and expensive procedures for handling and disposal. In addition, this method is non-selective, dangerous and, in many cases, does not effect complete removal of hydrocarbon gases.
Air flow through a heated metal catalyst (nickel, copper, zinc, cobalt, platinum and palladium) is an effective method of removing low molecular weight hydrocarbons. Hydrocarbons are oxidized to carbon dioxide and water in the presence of the catalyst. Metal catalysts are reusable and stable during extended use if provided with sufficient oxygen for regeneration and heated within specified operating temperatures. (High temperatures can damage certain metal catalysts and reduce their efficiency.) The amount of oxygen required for regeneration is stoichiometrically related to the levels of hydrocarbons removed (oxidized) from the air stream. For the trace amounts of hydrocarbons present in most air supplies (&lt;10 ppm) the amount of oxygen consumed is negligible. Levels of carbon dioxide produced under these conditions are also minimal.
The efficiency of any catalyst is largely determined by the amount of active surface area and control over catalyst temperature. The active surface area can be increased by coating a suitable fibrous support material, such as asbestos, with the metal catalyst. On more rigid porous support materials (such as aluminum oxide pellets) a greater active surface area can be achieved by impregnating with metal solutions containing organic solutes (fatty acids for example) that lower the surface tension and/or by infiltrating under partial vacuum. The organic solutes are then removed (oxidized) at high temperature before the catalyst is used. Both methods increase the penetration of the soluble metal into the support material thereby developing a greater active surface area.
Non rigid catalyst support materials such as asbestos are disadvantageous for systems with high flow rates. Furthermore, asbestos is a health hazard. High flow rates may compress these materials, which will impede air flow and reduce catalyst efficiency. Moreover, increasing the active surface area on more rigid support materials does not, in most cases, enhance catalyst efficiency sufficiently to provide 100% efficiency at high flow rates.
The combustion of a low molecular weight hydrocarbon (ethylene) by catalytic combustion in the presence of platinum is already well known, and reference may be made to U.S. Pat. No. 4,331,693, issued May 25, 1982 to Wojciechowski. In this patent, aluminum oxide is impregnated with a solution of a fatty acid followed by chloroplatinic acid, then dried and heated.
Other patents of interest which are related to various techniques of catalytic oxidation of carbon containing materials include U.S. Pat. Nos. 1,869,736; 3,025,132; 3,167,400; 3,654,432; 4,138,220; 4,867,949; and U.K. patents 953,216 and 2,065,629.